Respiratory motion can cause severe blurring in magnetic resonance imaging (“MRI”) studies of the thoracic or abdominal region of a subject (i.e., a person or other animal undergoing medical or scientific treatment or examination) when the total duration of the experiment is not short compared to the respiratory period. For such experiments, a variety of methods exist to reduce the effects of respiratory motion on the resulting images. Such methods can be broadly classified into four different types, modifying the subject of the image, nuclear magnetic resonance (“NMR”) based methods, direct non-NMR based methods, and indirect non-NMR based methods.
One method of minimizing motion that causes blurring is to modify the subject in one way or another. For example, the subject can be asked to hold their breath. Although this can minimize respiratory motion and eliminate blurring, it is not applicable to animal subjects and cannot be used with some subjects with respiratory or other illnesses. Subjects can also be intubated, or mechanically ventilated. This permits exact synchronization of MRI data acquisition to the respiratory cycle. However, this may induce significant reductions in cardiac output and liver blood flow compared to free breathing, and is an invasive procedure that is often not desirable for a relatively simple MRI procedure.
Certain NMR-based methods also exist that can minimize blurring of MRI images due to motion of the subject. One such method is called the navigator echo technique. The navigator echo technique is accomplished by acquiring a one-dimensional profile along the motion direction. This allows the respiratory phase to be measured at any given time. Once the respiratory phase has been determined, it can be used to produce artifact-free images, but this method is applicable only for simple motion that has a period that is long compared to the time required to acquire one phase-encoded step. Gradient moment nulling is another NMR based method for limiting or eliminating artifacts from motion. Gradient moment nulling eliminates the net evolution of nuclear spins moving in the magnetic field gradient by varying the amplitude and/or duration of the gradient. Gradient moment nulling, although applicable in human experiments, is an insufficient technique for small animal imaging.
There are also a number of direct non-NMR methods that are useful to eliminate blurring caused by motion. In particular, by detecting the respiratory motion of the subject, a gate signal can be generated that can be used in the collection and analysis of data from the MRI system by focusing only on the data collected while the subject's body is nearly motionless. Optical sensors can detect chest motion when placed on the chest of the subject. Motion can be detected for example by placing the fiber so that the motion causes the fiber to flex which interrupts the light propagation through the fiber. Optical fibers can also detect motion when the motion causes a variation in the distance of the chest to an infrared emitter/detector. Both methods of using optical fibers detect respiratory motion through monitoring of the absolute chest position. Such techniques are advantageous in that they do not require electrical leads inside the probe or magnet, but are limited because of the need for very careful placement and maintenance of the fiber on or near a specific part of the subject's chest. Another method of direct non-NMR detection of respiratory motion is through the use of a pickup coil. Pickup coils generate a signal through electromagnetic induction in a wire loop placed on the subject's chest within a magnetic field. In particular, as shown in FIG. 1, a wire 16 is looped around the abdomen of a subject 10 (here a mouse used for research) positioned on a patient handling bed (not shown) in a magnetic bore 12 of an MRI machine to form a pickup coil 18. The free ends of wire 16 are connected to a voltage measuring device 20 that is positioned outside of magnetic bore 12. When a magnetic field B 14 is applied by the MRI machine, the output from voltage measuring device 20 will provide a measure of the respiration of subject 10 when pickup coil 18 is positioned orthogonally to the direction of the magnetic field. Pickup coils are inexpensive to build and easy to use, but in some cases require wire leads to be placed within the radio frequency (“RF”) coil and gradients. Prior art systems using pickup coils had leads that introduced RF interference artifacts, posed a potential hazard of burns due to mutual inductance within the RF and/or gradients, and were subject to artifacts in the respiratory signal during scanning.
There are also indirect non-NMR based methods that can be used to minimize artifacts caused by motion. Many of these methods are based on the effects (on the subject and the immediate area surrounding the subject) of breathing. One such method utilizes a pressure detector on the chest of the subject with a pressure sensor outside the RF coil. Examples of such detectors are strain gauges, air bellows, or balloons. Although the theory behind these types of sensors is straightforward, they are quite sensitive to temperature variations, drifting baselines, and leakage. Also, they are generally not amenable to use on small animals. For example, a decrease in the heart rate of a mouse of up to 30% has been observed when using a pressurized pillow against its abdomen to detect respiration, which change is undesirable when using mice to conduct genetic or drug testing. The temperature and carbon dioxide content of exhaled air can also be used to monitor respiration, but the response is too slow for use in small, rapidly breathing animals. Another method that takes advantage of the effects of respiration is plethysmography. A plethysmograph utilizes an airtight chamber housing the subject, and uses a remote airflow sensor to detect motion of the subject. Although this type of sensor is quite useful in animals, it is quite expensive, complex and limits access to the animal. It is also highly unlikely, because of the sealed chamber, that such a method would be used with human subjects. Photoplethysmography, can also be used. Photoplethysmography detects respiratory and cardiac variations in superficial blood flow by infrared light scattering, but is again not amenable to imaging of small animals.
There are also methods that use certain characteristics of the MRI imaging process itself. For example, respiratory ordered phase encoding (ROPE) which is generally used along with a technique (either NMR or non-NMR based) to measure respiratory motion, can be used to generate artifact-free images, but requires specialized hardware and software, not generally available on animal imaging systems, to reconstruct the data. The data is acquired and processed with a mathematical algorithm that uses the respiratory phase signal to correct for the simple motion caused by respiration. Another method is the measurement of probe Q modulation, which allows for the detection of both respiratory and cardiac motion but requires special spectrometer hardware and can be prone to errors due to non-respiratory motion of the animal.
A number of patents have been directed towards methods of reducing image blurring due to motion. For example, U.S. Pat. No. 5,035,244 (Stokar), basically discloses an improvement on ROPE. It is a method that measures respiratory displacement data and uses that data to set the phase encoding gradient in order to minimize artifacts caused by motion. The important aspect of the invention is the mathematical algorithm that is utilized to select the phase encoded gradient strengths based on the respiratory displacement data. The disadvantages of this method are first, that a standard sensor, which has significant drawbacks, is necessary to obtain the respiratory displacement data, and second that it does not remedy the effects of cardiac motion.
U.S. Pat. No. 5,038,785 (Blakely, et al.) discloses a method of using electrodes to monitor the cardiac cycle and an expansion belt to monitor the respiratory cycle of a subject being imaged. During a MRI scan, noise signals or spikes are superimposed on the cardiac cycle signal. A noise spike detector detects spikes. Specifically, a comparator compares each signal received from the electrodes with properties of a cardiac signal, such as the slope. When the comparator determines that a noise signal is being received, it gates a track and hold circuit. The track and hold circuit passes the received signal except when gated by the comparator. When gated by the comparator, the track and hold circuit continues to supply the same output amplitude as in the beginning of the gating period. A filter then smoothes the plateaus in the cardiac signal formed as the noise signal signals are removed.
U.S. Pat. No. 5,427,101 (Sachs, et al.) discloses a method of reducing motion artifacts in MRI images through use of an algorithm. The method first acquires an initial set of data frames that includes a mechanism for indicating a relative position of each frame. The positional markers in these data frames are then evaluated and those that are deemed positionally worse are reacquired.
U.S. Pat. No. 5,729,140 (Kruger, et al.) teaches to a method for removing artifacts from NMR images by acquiring two data sets from which a desired image can be reconstructed, calculating the correlation between the two data sets to produce a correlation array, and producing a corrected image from the correlation array.
U.S. Pat. No. 6,073,041 (Hu, et al.) discloses a method for the removal of signal fluctuation due to physiological factors such as respiration and cardiac pulsations. The technique comprises simultaneous measurement of physiological motion during MRI data acquisition. Then in post processing steps, imaging data are retrospectively ordered into unit physiological cycles, after which the physiological effects are estimated and removed from the MRI data.
U.S. Pat. No. 6,088,611 (Lauterbur, et al.) teaches to a method for obtaining high-resolution snapshot images of moving objects in MRI applications through the elimination of ghosting and other image artifacts. The method works by estimating motion frequency data, estimating the amplitude data for the motion frequency data, interpolating the motion frequency data and the amplitude data to generate snap-shot data frames, and generating snapshot images of each snapshot data frame.
U.S. Patent Publication No. US 2001/0183611 A1 (Fishbein, et al.) discloses a method for measuring the respiration of a subject that uses a small electromagnetic pickup coil coupled to a mechanical lever to sense the respiratory and cardiac motion of a subject in a MRI scanner. It generates an electrical signal that is proportional to the velocity of motion which can be used to synchronize the MRI scanner to prevent blurring induced by motion during the MRI scan. The apparatus uses a complex mechanical linkage to couple the pickup coil to the subject.
Commercially available sensors, as well as the methods and systems discussed above, are either unreliable, unworkable in certain situations, or are too expensive. Therefore, there remains a need for a method of detecting respiratory motion that is reliable, amenable to different kinds of subjects and inexpensive.